Power MOSFET transistors find application as switches in AC circuits. The conduction channel of a MOSFET transistor is able to conduct AC current in a bidirectional manner as is required of an AC switch; however, a commonly used construction for MOSFET transistors results in a parasitic or "body diode" being formed in the device. The body diode appears as, in effect, a diode connected between the drain and source regions of the MOSFET transistor. In an N-channel MOSFET transistor, for example, the body diode appears in a polarity sense to allow conduction from the source region into the drain region that is, in a sense opposite to that which would be considered the "normal" direction of current flow through an N-channel MOSFET transistor.
MOSFET transistors are relatively inexpensive devices in comparison with other available devices for controlling current in AC power circuits, such as triacs, for example, and accordingly it is desirable to utilize them in that capacity. It is furthermore herein recognized to be desirable to achieve satisfactory operation as an AC power switch with a single MOSFET transistor.
When a MOSFET transistor is utilized for controlling current flow in an AC circuit, conduction in one polarity sense can be shut off by the normal action of the control gate electrode; however, conduction in the opposite polarity sense cannot be prevented by action of the control gate electrode because the conduction channel, even when made non-conductive, is bypassed or shunted by the body diode conducting and, in effect, the device as seen from its terminals, is not shut off.
Prior art techniques for utilizing MOSFET transistors in AC power switching applications have included utilizing a second MOSFET transistor to block the action of the body diode or else full wave bridge rectifier-type configurations have been used. As a result, extra semiconductor devices are required, thus increasing the cost so that no very significant advantage is obtainable in comparison with a triac, for example.